1. Field of the Invention
The present invention relates to an active matrix display device using a thin film transistor (hereinafter referred to as TFT) as a switching element, and more specifically, to a pixel structure of the active matrix display device.
2. Description of the Related Art
A liquid crystal display device with a method of performing driving by using TFTs (TFT driving method) is known as an active matrix display device. In the liquid crystal display device, a voltage applied to liquid crystal can be controlled for each pixel by using TFTs formed over a transparent substrate made of glass or the like, and thus, images are clear. Therefore, the liquid crystal display devices are widely used in OA equipment, TVs and the like.
FIG. 1 shows an equivalent circuit of one pixel in the liquid crystal display device with the TFT driving method. A pixel TFT 102 is arranged at an intersection portion of a gate signal line 100 and a source signal line 101. A gate terminal of the pixel TFT 102 is electrically connected with the gate signal line 100. One of input and output terminals of the pixel TFT 102 (a source or a drain terminal) is connected with the source signal line 101, and the other terminal is connected with a liquid crystal 103 and a storage capacitor 104.
When the pixel TFT 102 enters an ON state in response to a signal output to the pixel TFT 102 from the gate signal line 100, a potential of the source signal line 101 is written into the liquid crystal 103 and the storage capacitor 104, and an electric charge is stored. Even after the pixel TFT 102 enters an OFF state, the electric charge stored in the liquid crystal 103 and the storage capacitor 104 tries to hold the written potential. A necessary value of the storage capacitor 104 is determined in accordance with an off current, a holding time, a parasitic capacitor and the like of the pixel TFT 102 that becomes a switching element.
FIG. 2 shows a cross-sectional structure of one example of a conventional storage capacitor. The storage capacitor is formed by an active layer 201 and a capacitor wiring 203 formed from the same film as that for a gate wiring as electrodes, and a gate insulating film 202 as a dielectric, which is formed between the active layer 201 and the capacitor wiring 203. By using the gate insulating film 202 as the dielectric, the highly reliable and high-quality storage capacitor can be formed even with the thin thickness.
Further, it is desirable that the active matrix display device is provided with a light shielding film. FIG. 3 shows a cross-sectional structure of a pixel TFT provided with a light shielding film under the pixel TFT as one such example. The light shielding film 301 and an insulating film 302 are formed on a glass substrate 300, and an active layer 303, a gate insulating film 304, and a gate wiring 305, which are provided for forming the pixel TFT, are sequentially laminated thereon. The light shielding film prevents light leakage and improves contrast, and there is obtained an effect of reducing an off current of the pixel TFT by shielding the pixel TFT from light. When the off current of the pixel TFT is reduced, the display data holding characteristic is improved, thereby obtaining a satisfactory display.
As a method of improving display quality (image quality) of the conventional active matrix display device and attaining energy saving, miniaturization and high reliability of the display device, the following points are given.
The first point is that, in the active matrix display device, a capacitor element structure is obtained, which can secure a sufficient storage capacitor even if an area for one pixel is reduced with higher resolution. If each pixel is provided with a storage capacitor that can have a large capacitor, the display data holding characteristic is improved, and thus, a satisfactory display can be obtained.
The second point is that, in the active matrix display device, an aperture ratio is not reduced while a sufficient storage capacitor is secured. If each pixel has a high aperture ratio, utilization efficiency of light of backlight is improved. Thus, energy saving and miniaturization of the display device can be attained.
Further, light leakage is prevented and contrast is improved by arranging the light shielding film. In addition, the off current of the pixel TFT is reduced by shielding the pixel TFT from light, which leads to an improvement of the display data holding characteristic.
Demands for an improvement of performance of an active matrix display device such as for high precision (minuteness of a pixel TFT), securing a sufficient storage capacitor, a high aperture ratio, and a light shielding film, are opposed to each other in a meaning that one demand is satisfied while other demands are neglected. An object of the present invention is to improve performance of the active matrix liquid crystal display device while the above demands are satisfied.
The present inventors contrived the formation of a storage capacitor with the use of a light shielding film in order to satisfy these mutually opposing demands. Further, the present inventors proposed a method of forming a storage capacitor having a large capacitor without lowering an aperture ratio.
FIG. 4A is a cross sectional view showing an example in which a light shielding film and a capacitor are formed by extending source and drain regions of a pixel TFT. A light shielding film 401 and a dielectric (first insulating film) 402 are formed on a glass substrate 400. One of the source and drain regions of the pixel TFT, which is connected with a pixel electrode 409, is widened in order to secure a necessary storage capacitor to thereby form an active layer 403.
The light shielding film 401 has conductivity and may be connected at the outside of a pixel region so as to have a constant potential such as a COMMON potential or a power supply. In a case where the capacitor of the light shielding film 401 is sufficiently larger than the storage capacitor of the pixel, the light shielding film 401 does not have to be connected at a constant potential as long as the potential variation is sufficiently small. Thus, the storage capacitor is formed by the active layer 403 and the light shielding film 401.
FIG. 4B is a diagram in which, in addition to the storage capacitor formed by the light shielding film 401 and the active layer 403, a capacitor wiring 410 is formed to secure a storage capacitor having a larger capacity. A gate insulating film 404 is formed on the active layer 403, and a gate wiring 405 and the capacitor wiring 410 are simultaneously formed. The capacitor wiring 410 is connected at the outside of the pixel region so as to have a constant potential such as a COMMON potential or a power supply to thereby form a capacitor with the active layer 403. In this way, a larger storage capacitor is secured without lowering an aperture ratio. Further, in FIG. 4B, the gate insulating film formed under the capacitor wiring 410 is formed to be thin in order to enlarge the storage capacitor.
In FIGS. 4A and 4B, the capacitors are formed by the source and drain regions of the pixel TFT and the light shielding film 401 arranged under the regions. However, the capacitor is not needed with respect to the region connected with a source signal line. The reason for that is because a charge in writing in a video signal to the source signal line increases if the capacitor is formed to the source signal line. Thus, as shown in FIGS. 5A and 5B, a structure is proposed in which a light shielding film is provided in two layers so as not to form a capacitor with respect to the region connected with the source signal line.
FIG. 5A shows an example of two layers of the light shielding film. A first light shielding film 501 is formed on a glass substrate 500, an insulating film 502 is formed thereon for insulation, and a second light shielding film 503 is formed. In FIGS. 5A and 5B, the first light shielding film 501 shields one of source and drain regions of a pixel TFT, which is connected with a source signal line, from light in order not to have a capacitor with an active layer 505. Further, the first light shielding film 501 has a contact with a gate wiring 507 to be used as a gate signal line.
The second shielding film 503 shields one of the source and drain regions of the pixel TFT, which is connected with a pixel electrode, from light to form a capacitor with the active layer 505. The structure in which light is not incident on the active layer 505 is taken by both the first light shielding film 501 and the second light shielding film 503. FIG. 5B is a diagram in which a capacitor wiring 512 is provided in FIG. 5A.
Note that the portion not shielded by the light shielding film indicates the portion that can be seen from the substrate side. At least a channel portion (channel forming region), an LDD region, and an offset region under a gate electrode of the active layer 505 should be shielded from light by the first light shielding film 501 and the second light shielding film 503.